This invention relates to methods of joining aluminum surfaces by brazing. As herein used, the term "aluminum" embraces aluminum metal and alloys thereof.
It is known to join aluminum components by disposing an aluminum brazing alloy between or adjacent the faying surfaces (i.e. the surfaces to be joined), and heating the brazing alloy and the faying surfaces in appropriately assembled relation to a temperature (herein termed "brazing temperature") at which the brazing alloy melts while the components remain unmelted. Upon subsequent cooling, the brazing alloy forms a fillet or joint that bonds the faying surfaces. For assured selective melting of only the brazing alloy in the heating step, it is commonly preferred that the melting point of the brazing alloy be at least about 30.degree. to 40.degree.C lower than that of the metal of the components. An example of a suitable aluminum brazing alloy is an Al-Si eutectic composition, which melts at about 577.degree.C.
Frequently, one or both of the faying surfaces are pre-clad with a layer of aluminum brazing alloy. Such pre-clad articles (e.g. so-called brazing sheet) are, however, relatively costly, and in many instances it is preferred to provide the brazing alloy in some form other than a cladding. One alternative expedient heretofore proposed, and offering potential advantages of convenience and economy for a variety of brazing operations, is to apply the brazing alloy to or adjacent one or both faying surfaces in powdered or particulate form carried in a suitable liquid or pastelike vehicle. The present invention, in a specific sense, is particularly directed to improvements in brazing procedures including such use of powdered or particulate brazing alloy.
Although fluxless brazing procedures have been devised, their use is limited because of economic and other considerations arising from the special conditions and equipment required for successful practice of such procedures, It is, therefore, generally necessary to employ a flux in brazing aluminum, to remove the oxide ordinarily present on exposed metal surfaces (including brazing alloy surfaces) at the locality of the joint, as well as to promote flow of molten brazing alloy during the heating step, and, desirably, to inhibit further oxide formation. The material used as a flux must be capable of acting as a flux to dissolve and/or otherwise remove metal (e.g. aluminum) oxides at the brazing temperatures while remaining essentially inert with respect to aluminum at such temperatures. Since fluxes are usually reactive (i.e. capable of removing oxide) only when at least partially molten, fluxes for aluminum brazing should as a practical matter be partly or wholly molten at brazing temperatures, e.g. (in the case of use of the aforementioned Al-Si eutectic brazing alloy) at temperatures not substantially higher, and indeed preferably lower, than 577.degree.C.
Flux materials heretofore commercially employed in brazing aluminum have commonly been mixtures of predominantly chloride salts, with minor additives of fluoride in some cases. These fluxes are water-soluble and are corrosive to aluminum in the presence of water; hence flux residue must be removed from brazed assemblies by washing at the end of the brazing operation. The washing step is not only inconvenient, but often is also not fully effective to remove all residual flux, as is necessary to prevent corrosion.
For use with a powdered brazing alloy, it has heretofore been proposed to provide a particulate chloride-containing flux in mixture therewith, i.e. again in a suitable vehicle, for simultaneous application to the faying surfaces of aluminum components which are to be joined by brazing. The selection of the vehicle has been limited to relatively costly organic liquids in which the chloride-containing flux does not attack the brazing alloy particles.
It has also been proposed to mix a powdered aluminum brazing alloy with finely divided aluminum fluoride (AlF.sub.3), e.g. in an aqueous vehicle for coating surfaces of aluminum components prior to brazing. The utility of such procedures is limited, however, to so-called dip brazing, wherein the components and powdered brazing alloy are heated to brazing temperature while immersed in a molten salt bath. That is to say, mixtures of powdered brazing alloy and powdered aluminum fluoride cannot be used for furnace brazing, because the melting point of aluminum fluoride is far too high to enable it to serve as a flux, although when immersed in a molten salt bath, the aluminum fluoride dissolves therein and cooperates with other ingredients of the bath to provide fluxing action. As will be appreciated, for many types of brazing operations, dip brazing is unsuitable or at least uneconomical or inconvenient, as compared to furnace brazing, i.e. brazing operations wherein the assembly to be brazed is heated in a confined chamber, typically in an atmospheric of air or inert gas but in any event without immersion in a molten bath.
In the copending U.S. patent application of Eric Robert Wallace and Ernest William Dewing, Ser. No. 447,168 filed Mar. 1, 1974 for Joining of Metal Surfaces and assigned to the same assignee as the present application, there are disclosed fluxed consisting essentially of a mixture of potassium fluoaluminate complexes (typically KAlF.sub.4 and K.sub.3 AlF.sub.6) and essentially free of unreacted potassium fluoride. These fluxes are found to be especially advantageous for aluminum brazing, in that they are nonhygroscopic and leave no substantially water-soluble residue, and that at temperatures ranging upwardly from about 560.degree.C they are reactive (i.e. effective to strip oxides from metal surfaces) while being essentially inert with respect to the metal of the surfaces and performing other usual flux functions, e.g. promoting flow of brazing alloy and preventing subsequent oxide formation. In addition, these fluxes, characterized by the essential absence of unreacted potassium fluoride, are substantially water insoluble. The use of these fluxes, therefore, overcomes problems associated with the aforementioned commercial chloride-containing fluxes, such as the necessity for washing a brazed assembly to remove flux residue and the possibility of corrosion resulting from unremoved flux residue.
Among other uses for these fluxes, the aforementioned copending application discloses mixtures of the flux in powdered form with a powdered aluminum brazing alloy in a resinous vehicle for application to aluminum parts to be brazed. While such use is satisfactory for various specific purposes, in other cases a resinous vehicle is undesirable in that the resin may leave a carbonaceous residue that is difficult or impossible to remove.